The present invention relates to a delay time measurement apparatus for an optical element and, more particularly, to a delay time measurement apparatus for an optical element, which includes a wavelength dispersion measurement apparatus for measuring wavelength dispersion that occurs when light passes through an object to be measured such as an optical fiber or the like, and a polarization dispersion measurement apparatus for measuring polarization dispersion that occurs when light passes through the object to be measured.
As is well known, the velocity at which an optical signal propagates in an optical element, for example, an optical fiber, varies depending on the wavelength of the optical signal.
Hence, the pulse width (time duration) of a pulse waveform in an optical pulse signal output from a light source having a wavelength spread is broadened in the optical fiber.
Since the propagation frequency band of the optical fiber is inversely proportional to the pulse width, it finally influences the limitation on the propagation velocity of an optical signal.
Hence, the measurement of the propagation velocity (wavelength dispersion) in the optical fiber in units of wavelengths is a very important test item for the optical fiber.
Especially, since an ultra-fast optical signal beyond 100 Gbits/s, which will be used in the next generation large-capacity optical network, has a pulse width as narrow as several ps (pico seconds), and a large wavelength spread, wavelength dispersion of the optical fiber considerably influences optical transmission.
In a pulse generation technique as well, the generation ratio of high-quality pulses, i.e., transform-limited optical pulses largely depends on the wavelength dispersion of the optical fiber and, hence, wavelength dispersion measurement becomes a more important item.
AS the wavelength dispersion measurement methods, (a) time-resolved spectrometry, (b) a pulse method, (c) an interference method, (d) a difference method, (e) a phase difference method, and the like have been proposed.
Of these methods (a) to (e), the pulse method (b) and interference method (c), which are relatively frequently implemented, will be explained below.
The pulse method proposed by Jpn. Pat. Appln. ROKAI No. 6-174592 will be explained first using FIG. 12.
As shown in FIG. 12, white pulses which are output from a white pulse light source 1 and have a broad wavelength range are wavelength-limited to a specific wavelength by a tunable optical bandpass filter 2, and are divided into an input optical pulse 4 and reference optical pulse 5 by an optical power divider 3.
The input optical pulse 4 enters one end of an optical power coupler 7 via a fiber 6 to be measured.
On the other hand, the reference optical pulse 5 directly enters the other end of the optical power coupler 7.
The optical power coupler 7 outputs a combined optical signal 8 obtained by combining the input optical pulse 4 and reference optical pulse 5 to a delay time detection means 9.
The delay time detection means 9 calculates a delay time tD of the input optical pulse 4 with respect to the reference optical pulse 5 on the basis of the combined optical signal 8.
More specifically, since the input optical pulse 4 is delayed when it has passed through the fiber 6 to be measured, two peaks form in the signal waveform of the combined optical signal 8 upon combining the input optical pulse 4 and the reference optical pulse 5 free from any time delay.
The time difference between these two peaks is the delay time tD detected by the delay time detection means 9.
By changing a wavelength xcex of the tunable optical bandpass filter 2, the delay time detection means 9 can calculate delay times tD(xcex) at individual wavelengths xcex.
The wavelength dependence of these delay times tD(xcex) defines wavelength dispersion characteristics.
The pulse method proposed by Jpn. Pat. Appln. KOKAI No. 4-177141 will be explained below using FIG. 13.
As shown in FIG. 13, optical pulses output from an ultra short pulse generation device 11 pass through an optical fiber 12 to be measured, and are then divided into two optical pulses A and B by an optical power divider 13.
Only a specific wavelength component of one optical pulse A passes through a tunable bandpass filter 14 as a first optical pulse.
On the other hand, the other optical pulse B passes through a delay line 15 as a second optical pulse.
These first and second optical pulses are combined by an optical power coupler 16, and the combined pulse is converted into an electrical signal by a photosensor 17.
The electrical signal is input to a pulse waveform observation device 18 to measure the relative delay time difference between the first and second optical pulses as a function of the wavelength, thus obtaining the aforementioned wavelength dependence of the delay time.
A wavelength dispersion measurement apparatus which uses the interference method (c) and is specified by JIS c6827, as shown in FIG. 14, is known as an apparatus that implements wavelength dispersion measurement with high precision.
As shown in FIG. 14, white light which is output from a white light source 20 and has a broad wavelength range is input to a spectroscope 21 having predetermined spectrum characteristics to extract the component of a specific wavelength xcexC.
The spectrum characteristics of the spectroscope 21 have a predetermined wavelength spread having a center wavelength xcexC, as shown in FIG. 15.
In the spectrum characteristics of this spectroscope 21, the width 1/e (e: base of natural logarithm) below the peak value is called a half-width.
In this case, the half-width is set to fall within the range from 2 to 10 nm.
Light output from the spectroscope 21 is split into input light A1 and reference light B1 by a beam splitter 22.
This input light A1 enters an object 23 to be measured such as an optical fiber or the like.
The input light A1 via the object 23 to be measured is launched into one end of a optical power coupler 26 comprising a half mirror as output light A2.
On the other hand, the reference light B1 is delayed a predetermined period by an optical path delay element 24, and then passes through a variable optical delay device 25 comprising a corner cube mirror, which is controlled to move in the direction in which light travels. The light output from the delay device 25 is launched into the other end of the optical power coupler 26 as reference light B2.
The optical power coupler 26 outputs combined light C obtained by combining the output light A2 and reference light B2 to a photosensor 27.
Note that a lock-in amplifier 28 is provided to amplify with high S/N only a signal output from the photosensor 27, which is synchronized with an optical chopper incorporated in the spectroscope 21.
In this case, if the output light A2 and reference light B2 have an equal optical path length, the light intensity of the combined light C, i.e., an interference intensity I, increases, and a large signal is output from the photosensor 27.
Therefore, the delay amount in the variable optical delay device 25 is adjusted to maximize the signal output from the photosensor 27, i.e., to match the optical path lengths of the output light A2 and reference light B2.
In this case, the delay amount of the reference light B2 from the reference light B1, i.e., the input light A1 in the optical path delay element 24 and variable optical delay device 25 is known.
Hence, the delay amount of the reference light B1 at that time is that of the output light A2, and the delay time of the object 23 to be measured can be measured from this delay amount.
FIG. 16 is a graph showing the relationship between the interference intensity I between the output light A2 and reference light B2, and an optical path difference L (=|L1xe2x88x92L2|) between the output light A2 and reference light B2.
Note that the optical path L2 of the reference light B2 is a total of an optical path L2a of the optical path delay element 24 and an optical path L2b of the variable optical delay device 25.
In this manner, the delay time of the object 23 to be measured can be obtained from the maximum optical path difference L of the interference intensity I, which is obtained by changing the optical path difference L in turn.
FIG. 17 is a graph showing a shift of the peak position of the interference intensity I when the center wavelength xcexC of the reference light B1 extracted by the spectroscope 21 is changed from xcex1 to xcex2.
A wavelength dispersion value can be obtained from a time corresponding to this shift amount xcex94Lxe2x80x20 of the peak position.
The aforementioned variable optical delay device 25 comprising a corner cube mirror can control the delay amount on the 1-xcexcm order (about 0.003 ps in time) by an external control signal.
Therefore, the wavelength dispersion measurement apparatus, which uses the interference method (c) and is specified by JIS c6827, as shown in FIG. 14, can measure the delay time with higher precision compared to the pulse method (b) and, for example, the wavelength dispersion of a low-dispersion object to be measured such as an optical fiber as short as several meters or the like can be measured.
As another important characteristic of an optical communication medium such as an optical fiber or the like, polarization dispersion characteristics are known.
More specifically, in an optical fiber which has an ideal circular section, an optical pulse signal that travels through this optical fiber does not suffer any propagation velocity difference independently of its direction in the sectional shape.
However, if an optical fiber does not have a circular but an elliptic sectional shape, or if an optical fiber is bent and its sectional shape locally has a lower profile, a propagation velocity difference is produced depending on the directions of polarization.
Hence, the measurement of the velocity differences of an optical signal that travels through the optical fiber in the respective directions of polarization, i.e., the propagation velocities (polarization dispersions) in the optical fiber in units of directions of polarization, is also a very important performance test item for the optical fiber.
FIG. 18 is a schematic block diagram showing the arrangement of a conventional polarization measurement apparatus using the interference method.
As shown in FIG. 18, a laser beam which is output from a laser source 29 and has a broad spectrum is converted into circularly polarized light by a quarter-wave plate 30, and the circularly polarized light is split into input light A3 and reference light B3 by a beam splitter 31.
These input light A3 and reference light B3 are respectively controlled by polarizers 32a and 32b to be linearly polarized light beams, the directions of polarization of which have a 90xc2x0 difference therebetween.
The optical path difference between the input light A3 and reference light B3 is set by a fixed optical delay device 33 and variable optical delay device 34.
Combined light C1 of the input light A3 and reference light B3 combined by the beam splitter 31 is input to a half-wave plate 35 so as to maintain orthogonal the directions of polarization of the input light A3 and reference light B3 which form the combined light C1, and is then input to an analyzer 37 via an object 36 to be measured.
This analyzer 37 extracts only a specific polarization component from the combined light C1 of the input light A3 and reference light B3, and outputs the extracted component to a photosensor 38.
The object 36 to be measured receives the combined light C1, in which the directions of polarization of the input light A3 and reference light B3 are maintained to be orthogonal.
Then, the analyzer 37 extracts the specific polarization component including polarization dispersions produced in the object 36 to be measured, and the extracted component is received by the photosensor 38.
At this time, when polarization dispersions having different propagation velocities have occurred in units of directions of polarization in the object 36 to be measured, the photosensor 38 measures the interference intensity between the input light A3 and the reference light B3, which has a direction of polarization perpendicular to that of the input light A3, and is delayed by the variable optical delay device 34.
In this case, a piezoelectric element (PZT) 40 mounted on the variable optical delay device 34 is used to help easily find an interference intensity peak by continuously slightly vibrating variable optical delay device 34.
Also, a displacement meter 40a is used to measure the spatial optical path length of the variable optical delay device 34.
The output from the photosensor 38 and that from the displacement meter 40a are supplied to a controller 39.
With this arrangement, since the photosensor 38 can easily find an interference intensity peak, and the displacement meter 40a can measure the optical path length with high precision, the controller 39 calculates changes in delay amounts in units of directions of polarization of the input light A3 via the object 36 to be measured using the reference light B3 as a reference direction of polarization.
Hence, the photosensor 38 measures the interference intensity between the input light A3 and the reference light B3, which has a direction of polarization perpendicular to that of the input light A3, and is delayed by the variable optical delay device 34.
However, even the aforementioned measurement methods have the following problems to be solved.
The wavelength dispersion measurement method shown in FIG. 13 is not influenced by a change in optical path difference of the optical fiber to be measured due to external factors such as changes in temperature, and the like, since a single optical pulse that has passed through the optical fiber 12 to be measured is divided into two pulses, and the delay times for the components of the respective wavelengths are measured using one of the two divided pulses as reference light on the time axis.
However, since the tunable bandpass filter 14 extracts a specific wavelength from the output light of the ultra-short pulse light source 11, the pulse width (time duration) of the optical pulse that has passed through the tunable bandpass filter 14 inevitably broadens with the existing technique due to limitations on the frequency band.
For this reason, in the wavelength dispersion measurement method shown in FIG. 13, it is difficult to identify the pulse peak position, and many measurement errors may be contained.
For example, if the pass wavelength width is 0.1 nm, the pulse width (time duration) of the extracted optical pulse is assumed to be not less than at least 20 ps (pico seconds).
Also, the wavelength dispersion measurement method shown in FIG. 13 uses the pulse waveform observation device 18 which comprises, e.g., an electric sampling oscilloscope, as a means for measuring the delay time difference.
Therefore, the wavelength dispersion measurement method shown in FIG. 13 is effective for measuring the dispersion of a long optical fiber (several km or more), but is not suitable for measuring low dispersion of an optical fiber as short as about 20 m, which are the typical unit length of an optical fiber (EDF: Erbium Doped Fiber) used in an EDFA (Erbium Doped Fiber amplifier).
On the other hand, the wavelength dispersion measurement method shown in FIG. 12 can implement measurements with higher precision than that shown in FIG. 13, by combining the white pulse light source 1 which outputs a short pulse light group over a continuous, broad wavelength range so as to measure the wavelength dispersion with high precision, and the delay time difference measurement means 9 which comprises, e.g., a streak camera or the like.
Since the spectrum width of the white pulse light source 1 is as broad as 200 nm, a tunable bandpass filter 2 having a bandwidth of around 1 nm can be inserted, and optical pulses having a pulse width (time duration) of several ps can be sufficiently obtained, thus easily identifying the peak position.
However, in the wavelength dispersion measurement method shown in FIG. 12, when a streak camera is used as the delay time difference detection means 9, the precision in the time domain is 0.3 ps or higher, which is insufficient to measure the wavelength dispersion of a low-dispersion object to be measured such as a short optical fiber or the like.
In the wavelength dispersion measurement apparatus using the interference method shown in FIG. 14, since the white light source 20 used as a light source emits continuous light, the strongest interference intensity I is obtained at a position where the difference between the delay amount of the output light A2 produced in the object 23 to be measured, and the sum delay amount of the reference light B2 by the optical delay element 24 and variable optical delay device 25 is xe2x80x9c0xe2x80x9d, i.e., the optical path difference L=|L1xe2x88x92L2| therebetween becomes xe2x80x9c0xe2x80x9d.
As described above, since the variable optical delay device 25 comprising a corner cube mirror can control the delay amount on the 1-xcexcm order, but its delay amount variable range is limited due to dimensional limitations, not so large a delay amount can be set.
Note that the optical delay element 24 may set a fixed delay amount, and this fixed delay amount may be added to the delay amount of the variable optical delay device 25 as a bias delay amount.
However, an optical fiber or the like must be used to set a large delay amount by the optical delay element 24, and it is very difficult for the element 24 to set a large delay amount with high precision like the variable optical delay device 25.
For this reason, since the measurement range is limited within the absolute delay amount given by the optical delay element 24 and variable optical delay device 25, the measurable length range of the object 23 to be measured is around several meters in case of, e.g., an optical fiber, as described in JIS c6827.
Hence, the conventional wavelength dispersion measurement apparatus using the interference method can measure the wavelength dispersion with higher precision than those using other methods, but since the absolute delay amount in the object to be measured poses a problem, it is impossible to measure the wavelength dispersion of an optical fiber having a length of 20 m, 50 m, or the like with high precision.
The same applies to the polarization dispersion measurement apparatus using the pulse method and that using the interference method shown in FIG. 18.
Note that the wavelength and dispersion measurement apparatuses are used to measure and evaluate the delay time of an optical element such as an optical fiber or the like as the object to be measured. Hence, the development of a delay time measurement apparatus for an optical element, including a wavelength dispersion measurement apparatus which can measure the wavelength dispersion of even a long object to be measured with high precision without being influenced by the absolute delay amount produced by the physical length of an optical element such as an optical fiber or the like as the object to be measured, and can measure wavelength dispersion with high precision over a broad length range from several meters to several ten meters, and a polarization dispersion measurement apparatus which can measure the polarization dispersion of a low-dispersion object to be measured with high precision using the same method as that of the wavelength dispersion measurement apparatus, has been strongly demanded.
The present invention has been made in consideration of the aforementioned situation, and has as its object to provide a delay time measurement apparatus for an optical element, including a wavelength dispersion measurement apparatus which can measure the wavelength dispersion of even a long object to be measured with high precision using pulse light as input light to be input to the object to be measured and reference light in place of continuous light without being influenced by the absolute delay amount produced by the physical length of an optical element such as an optical fiber or the like as the object to be measured, and can measure wavelength dispersion with high precision over a broad length range from several meters to several ten meters, and a polarization dispersion measurement apparatus which can measure the polarization dispersion of a low-dispersion object to be measured with high precision using the same method as that of the wavelength dispersion measurement apparatus.
In order to achieve the above object, according to one aspect of the present invention, there is provided a delay time measurement apparatus for an optical element, comprising:
a pulse light source which can vary a wavelength of light to be output, and outputs an optical pulse having a predetermined repetition period;
wavelength setting means for setting a wavelength of light to be output from the pulse light source;
an optical power divider for dividing the optical pulse output from the pulse light source into a first optical pulse and a second optical pulse to be input to an optical element as an object to be measured;
optical delay means capable of changing a spatial optical path length along which the first optical pulse divided by the optical power divider travels;
control means for changing the spatial optical path length of the optical delay means; and
detection means for receiving a measurement optical pulse output from the optical element as the object to be measured, and a reference optical pulse output from the optical delay means, and detecting a delay time of light that has passed through the optical element as the object to be measured from a change in spatial optical path length required for superposing the measurement and reference optical pulses on each other.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.